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### Re^2: A bad shuffle

 on Mar 21, 2005 at 01:43 UTC ( #441099=note: print w/replies, xml ) Need Help??

in reply to Re: A bad shuffle

Actually, the original "naive_shuffle" is not a Fisher-Yates shuffle, it is a "naive shuffle" implementation. The OP's analysis is correct.

His final algorithm, however, is a correct Fisher-Yates implementation.

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Re^3: A bad shuffle
by jdporter (Canon) on Mar 21, 2005 at 03:29 UTC

perlfaq4: How do I shuffle an array randomly? not only gives a "canonical" implementation of the Fisher-Yates algorithm in Perl, but also refers to the List::Util module's shuffle function, which is an implementation of Fisher-Yates in C. The algorithm is also implemented in Abigail's Algorithm::Numerical::Shuffle, the doco of which gives some citations into the literature (Knuth, Fisher&Yates, etc.).

The Fisher-Yates has also been discussed many times on clpm. (You can do a Google Groups search. My ego compels me to link to this posting by yrs trly, which isn't about F-Y, but uses it.)

Yes, I was looking at the code for List::Util earlier today. In addition to the C implementatin of the Fisher-Yates shuffle, it includes the following "backup" in Perl:

sub shuffle (@) { my @a=\(@_); my \$n; my \$i=@_; map { \$n = rand(\$i--); (\${\$a[\$n]}, \$a[\$n] = \$a[\$i])[0]; } @_; }
Pretty gnarleous, IMO. Kind of like a hybrid between F-Y and Tanktalus's shuffler.

The C implementation of List::Util::shuffle is 10-20x faster than the Perl implementation. For the practical programmer: end of story. Still, to satisfy my monkly preoccupation with how many angels can lambada on the head of a pin, and more importantly, in order to reduce this dead horse to a thin protein film, I benchmarked the three shufflers: ta = Tanktalus's shuffler; lu = Perl implementation of List::Util::shuffle; rp = random_perm:

N = 1000 Rate rp lu ta rp 230/s -- -10% -22% lu 257/s 12% -- -13% ta 296/s 29% 15% -- N = 10_000 Rate ta lu rp ta 14.3/s -- -27% -34% lu 19.6/s 37% -- -10% rp 21.7/s 52% 11% -- N = 100_000 Rate ta lu rp ta 0.144/s -- -92% -93% lu 1.75/s 1118% -- -13% rp 2.01/s 1302% 15% --

the lowliest monk

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